The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine. In particular, it relates to a lean NOx catalyst.
It is well known in the art to use catalyst compositions, including those commonly referred to as three-way conversion (xe2x80x9cTWCxe2x80x9d) catalysts to treat the exhaust gases of internal combustion engines. Such catalysts, containing precious metals like platinum, palladium, and rhodium, have been found both to successfully promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and to promote the reduction of nitrogen oxides (NOx) in exhaust gas, provided that the engine is operated around balanced stoichiometry for combustion (xe2x80x9ccombustion stoichiometryxe2x80x9d; i.e., between about 14.7 and 14.4 air/fuel (A/F) ratio).
However, fuel economy and global carbon dioxide (CO2) emissions have made it desirable to operate engines under lean-burn conditions, where the air-to-fuel ratio is somewhat greater than combustion stoichiometry to realize a benefit in fuel economy. Diesel and lean-burn gasoline engines generally operate under highly oxidizing conditions (i.e., using much more air than is necessary to burn the fuel), typically at air/fuel ratios greater than 14.7 and generally between 19 and 35 Under these highly lean conditions, typical three-way catalysts exhibit little activity toward NOx reduction, as their reduction activity is suppressed by the presence of excess oxygen.
The control of NOx emissions from vehicles is a worldwide environmental problem. Lean-bum, high air-to-fuel ratio, and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Development of an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions accordingly is critical.
Recently, copper-ion exchanged zeolite catalysts have been shown to be active for selective reduction of NOx by hydrocarbons in the presence of excess oxygen. Platinum-ion exchanged zeolite catalyst is also known to be active for NOx reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the lightoff temperature of hydrocarbon oxidation. All the known lean-NOx catalysts reported in the literature tend to lose their catalytic activity for NOx reduction when the catalyst temperature reaches well above the lightoff temperature of hydrocarbon oxidation. This narrow temperature window of the lean-NOx catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult for lean-burn gasoline or diesel engines.). As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome all three problems simultaneously before they can be considered for commercial use. In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NOx catalysts promote the conversion of such compounds to SO2 and SO3 during combustion. Such SO2 will adsorb onto the precious metal sites at temperatures below 300xc2x0 C. and thereby inhibits the catalytic conversions of CO, CxHy (hydrocarbons) and NOx. At higher temperatures with an Al2O3 catalyst carrier, SO2 is converted to SO3 to form a large-volume, low-density material, Al2(SO4)3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
Another alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia or urea as a co-reductant. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx).
Yet another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-bum gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable non-selective catalytic reduction xe2x80x9cNSCRxe2x80x9d catalysts that work with hydrocarbon co-reductant in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
A leading catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, commonly called the xe2x80x9clean-NOx trapxe2x80x9d. The lean-NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus xe2x80x9ctrappedxe2x80x9d, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoichiometrically fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
The lean-NOx-trap technology has been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. Such presence of SO3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed the use of selective SOx adsorbents upstream of lean NOx trap adsorbents. Furthermore, catalytic oxidation of NO to NO2 is limited in its temperature range. Oxidation of NO to NO2 by a conventional Pt-based catalyst maximizes at about 250xc2x0 C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NOx trap technology with respect to temperature and sulfur considerations.
Another NOx removal technique comprises a non-thermal plasma gas treatment of NO to produce NO2 which is then combined with catalytic storage reduction treatment, e.g., a lean NOx trap, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. In the lean NOx trap, the NO2 from the plasma treatment is adsorbed on a nitrate-forming material, such as an alkali material, and stored as a nitrate. An engine controller periodically runs a brief fuel-rich condition to provide hydrocarbons for a reaction that decomposes the stored nitrate into benign products such as N2. By using a plasma, the lean NOx trap catalyst can be implemented with known NOx absorbers, and the catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the nitrate to N2. Accordingly, an advantage is that a method for NOx emission reduction is provided that is inexpensive and reliable. The plasma-assisted lean NOx trap can allow the life of precious metal lean NOx trap catalysts to be extended for relatively inexpensive compliance to NOx emission reduction laws. Furthermore, not only does the plasma-assisted lean NOx trap process improve the activity, durability, and temperature window of lean NOx trap catalysts, but it allows the combustion of fuels containing relatively high sulfur contents with a concomitant reduction of NOx, particularly in an oxygen-rich vehicular environment.
What is needed in the art is an exhaust gas catalyst system having improved durability, as well as effective NOx management, over extended operating time. The present invention overcomes many of the shortcomings of the prior art.
Alkali metal oxides, such as lithium oxide, generally are not very good catalysts for NOx treatment, and, additionally, are very susceptible to sulfate poisoning. Alkaline earth oxides, such as barium oxide, generally are good NOx catalysts, but, they also are susceptible to sulfate poisoning. However, alkali metal sulfates generally have a low temperature of decomposition. In particular, lithium sulfate, of all the alkali or alkaline earth sulfates, has the lowest temperature of decomposition. Lithium sulfate formed on a catalyst bed decomposes at xcx9c800xc2x0 C. in an oxidizing atmosphere. In contrast, barium sulfate, of all the alkali or alkaline earth sulfates, has the highest temperature of decomposition, at about 1600xc2x0 C. in an oxidizing atmosphere.
Some lean burn applications, such as diesel engines, are never xe2x80x9crichxe2x80x9d. An always lean system cannot generate an exotherm from hydrocarbon combustion and seldom exceeds about 800xc2x0 C. on a catalyst bed. Alkaline earth sulfates, and even most alkali metal sulfates, accordingly would not typically be expected to ever decompose once formed on an always lean system. Nevertheless, lithium sulfates may decompose even at the temperatures achievable on an always lean system.
According to the present invention, an alkali metal oxide is included into the catalyst formulation of a NOx occluding catalyst. The alkali metal oxide provides a preferential scavenger for sulfates deposited on the catalyst surface. The alkali metal sulfate can be decomposed during high temperature excursions. The alkali metal sulfate sulfate decomposes to lithium oxide and sulfur is released to the exhaust stream as SO2. In this manner, irreversible sulfation of the active NOx agent and ozone agent is thereby prevented.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, and appended claims.
The catalyst generally comprises a calcined coating of an alkali metal oxide and an alkaline earth/support oxide coated onto a chemically stable and thermally insulating substrate, typically such as cordierite, mullite, and the like. The support oxide generally may be a zeolite or an aluminum oxide or a mixture of zeolite and aluminum oxide. Preferably, the catalyst coating also includes a ceramic oxide binder and/or stabilizer.
The alkali metal oxide catalyst component may comprise any alkali metal. Preferably, the alkali metal oxide catalyst component comprises an alkali metal selected from the group consisting of potassium, sodium, lithium, and mixtures thereof. Sodium and lithium and mixtures thereof are more preferred; lithium is particularly preferred.
Suitable lithium compounds for introduction of the alkali metal component include lithium formate, lithium citrate, lithium acetate, lithium oxylate, lithium nitrate, lithium carbonate, lithium hydroxide and lithium oxide. Lithium acetate, lithium oxylate, and lithium formate are preferred. Lithium formate is particularly preferred.
Suitable sodium compounds for introduction of the alkali metal component include sodium formate, sodium citrate, sodium acetate, sodium oxylate, sodium nitrate, sodium carbonate, sodium hydroxide and sodium oxide. Sodium acetate, sodium oxylate, and sodium formate are preferred. Sodium formate is particularly preferred.
Suitable potassium compounds for introduction of the alkali metal component include potassium formate, potassium citrate, potassium acetate, potassium oxylate, potassium nitrate, potassium carbonate, potassium hydroxide and potassium oxide. Potassium acetate, potassium oxylate, and potassium formate are preferred. Potassium formate is particularly preferred.
The alkaline earth/support catalyst component preferably is not more than about 95 wt % of the total catalyst, and more preferred at not more than about 90 wt %. Particularly preferred is alkaline earth/support component at about 85 wt %. Preferably, the alkali metal oxide catalyst component is at least about 4 wt % and more preferred at least about 8 wt %. Particularly preferred is alkali metal oxide at about 12 wt %. The ceramic binder preferably is greater than about 1 wt % and not more than about 7 wt %; about 3 wt % is particularly preferred.
A preferred calcined catalyst coating comprises less than about 49 wt % alkaline earth/zeolite, less than about 42 wt % alkaline earth/aluminum oxide, more than about 6 wt % alkali metal oxide and not more than about 7 wt % ceramic oxide binder. A more preferred calcined coating comprises less than about 48 wt % alkaline earth/zeolite, less than about 41 wt % alkaline earth/aluminum oxide, more than about 8 wt % alkali metal oxide, and not more than about 5 wt % ceramic oxide binder. A particularly preferred calcined coating comprises about 47 wt % alkaline earth/zeolite, about 40 wt % alkaline earth/aluminum oxide, about 10 wt % alkali metal oxide and about 3 wt % ceramic oxide binder.
The surface area of the alkali metal oxides may be stabilized with other binder oxides, such as oxides of silicon, titanium, and/or zirconium. Titanium and zirconium are preferred; zirconium is particularly preferred. The binder/stabilizer preferably comprises not more than about 7 wt %; more preferred at not more than about 5 wt %; and, particularly preferred not more than about 3 wt %.
Suitable zirconium compounds for introduction of the surface area stabilizer include zirconium acetylacetonate, zirconium n-butoxide, zirconium nitrate, zirconium tetraamine nitrate, zirconium tetraamine citrate, zirconium 2-ethylhexanoate, and zirconium isopropoxide. Zirconium tetraamine citrate, zirconium butoxide, and zirconium isopropoxide are preferred; zirconium butoxide and zirconium isopropoxide are more preferred; and, zirconium isopropoxide is particularly preferred.
Suitable titanium compounds for introduction of the surface area stabilizer include titanium oxylate, titanium ethoxide, titanium methoxide, titanium isopropoxide, zirconium n-butoxide. Titanium butoxide, titanium ethoxide, and titanium isopropoxide are preferred; titanium ethoxide and titanium isopropoxide are more preferred; and, titanium isopropoxide is particularly preferred.
Suitable silicon compounds for introduction of the surface area stabilizer include silicon acetate, tetraethoxysilane, tetramethoxysilane, vinyltrimethoxysilane, ureidopropyltriethoxysilane and aminopropylsilanetriol. Tetraethoxysilane, silicon acetate, and tetramethoxysilane are preferred; silicon acetate and tetramethoxysilane are more preferred; and, tetramethoxysilane is particularly preferred.
The catalyst of the present invention comprises an alkaline earth/support component, wherein the support may be a zeolite and/or alumina. The alumina fraction of the support may be present in a proportion ranging from about 0 wt % to about 100 wt % of the alkaline earth/support fraction. A particularly preferred admixture comprises an alkaline earth/aluminum oxide fraction in a proportion ranging from about 35 wt % to about 45 wt %, and an alkaline earth/zeolite fraction in a proportion ranging from about 55 wt % to about 65 wt %.
Any type zeolite may be used; preferred zeolites include X type zeolite, Y type zeolite, and/or ZSM-5 type zeolite. A ZSM-5 zeolite with pores 4-5 A (angstroms) is preferred and an X zeolite is more preferred and a Y-type zeolite with pores 7-8 A is greatly preferred. A zeolite surface area of at least about 300 m2/gram is preferred, at least about 400 m2/g is more preferred, and a surface area of at least about 500 m2/g is particularly preferred. The preferred zeolite average particle size is less than about 0.9 microns; more preferred are zeolite particles of an average size less than about 0.6 microns; and particularly preferred are zeolites having an average particle size less than about 0.3 microns. The zeolite particles preferably feature average pore sizes ranging from about 4 to about 10 angstroms (xe2x80x9cAxe2x80x9d), with average pore sizes ranging from about 7 to 8 A particularly preferred. Preferably, the zeolite has a Na2O content of at least about 3 wt %; a content of at least about 8 wt % is more preferred; and a content of at least about 13 wt % is particularly preferred.
The aluminum oxide particles providing the matrix comprise agglomerations of small psuedocrystalline alumina particles typically of about 0.3 microns or less. The agglomerations preferably are larger than about 10 microns and less than about 30 microns. The alumina matrix is mixed with a Ba-zeolite catalyst component, typically featuring a particulate size of about 0.3 microns. In general, the smaller the zeolite particle, the more hydrothermally stable the zeolite becomes, thereby improving its long-term activity. Pursuant to the invention, the zeolite active catalysts preferably are dispersed throughout the alumina matrix.
Preferably, the alumina has a surface area of at least about 150 m2/gram; a surface area of at least about 200 m2/g is more preferred; and, a surface area of at least about 250 m2/g is particularly preferred. An average alumina pore size of at least about 40 A is preferred; a pore size of at least about 60 A is more preferred; and, a pore size of at least about 80 A is particularly preferred. Specific acidity ranging from about 50 moles (millimoles) n-butylamine/m2xc3x9710xe2x88x924 to about 500 mmoles n-butylamine/m2xc3x9710xe2x88x924 is preferred; a specific acidity of about 350 mmoles n-butlyamine/m2xc3x9710xe2x88x924 is particularly preferred.
The zeolite catalyst component preferably comprises an alkaline earth content of about 18 wt % to about 36 wt %; about 24 wt % to about 30 wt % is particularly preferred. The aluminum oxide catalyst component preferably comprises an alkaline earth content of about 18 wt % to about 36 wt %; about 24 wt % to about 30 wt % is particularly preferred.
Any alkaline earth element may be used as the active catalyst. For example, the active catalyst element may comprise calcium, strontium, and/or barium. The use of barium is particularly preferred. Tests have indicated that calcium-doped catalysts generally convert about 30% NOx to N2; strontium-doped catalysts generally convert about 50% NOx to N2; barium-doped catalysts generally convert about 70% NOx to N2. Based on a desire in the industry to optimize conversion of NOx to N2 at about 90% or better, barium is the particularly preferred occluding catalyst material.
Accordingly, Ba is the preferred occluding catalyst for both the zeolite and the alumina components of the present catalyst. The alumina matrix features enhanced trapping efficiency if it is doped with a material such as barium. In specific, nitrogen species can neutralize catalytically active sites. Barium is the most robust alkaline earth element for resistance to nitrogen poisoning. The alumina matrix provides sacrificial sites for nitrogen poisoning, thus precluding large polycyclic nitrogen compounds from entering and poisoning the small pores of the zeolite catalyst component. The zeolite catalyst component preferably comprises a barium content of about 18 wt % to about 3,6 wt %; about 24 wt % to about 30 wt % is particularly preferred. The alumina matrix catalyst component preferably has a barium content of at least about 14 wt %; at least about 21 wt % is more preferred; and, at least about 28 wt % is particularly preferred. Exhaust deposits of oil derived xe2x80x9cglassyxe2x80x9d compounds such as calcium phosphate and zinc phosphate can greatly reduce diffusion. High levels of alkaline earths, such as barium, prevent formation of these diffusion limiting barriers.
Suitable barium sources for preparation of a barium-alumina component include barium nitrate, barium acetate, barium hydroxide, barium ethoxide, barium isopropoxide, and/or barium 2-ethylhexanoate. Barium acetate, barium isopropoxide, and barium 2-ethylhexanoate are preferred. Barium 2-ethylhexanoate is particularly preferred.
The following examples are provided to further describe the invention. The examples are intended to be illustrative in nature and are not to be construed as limiting the scope of the invention.